Seems the more I learn things, the more I see a convergence. I see the MVC style of project development gaining popularity in both PHP and .NET apps. jQuery seems to have become the standard for most sites, be they .NET or PHP. And CSS is slowly gaining a consistent balance between multiple different browsers.
In all honestly, I don't feel as torn today as I did years ago when I first started learning contrasting technologies. The more I learn, the more I feel comfortable enough jumping between languages.
The only pressure I feel today is in the environment I would like to work in. If I want a big corporate job, my chances are better going with .NET/C#. That isn't to say you can land a nice-paying job working with Open-Source technologies - you can, and I have. Either way, you're going to wind up learning the central tenets of web-development regardless what language you land on.
Learn all you can, and can all you learn ;)